Are deep eutectic solvents a real alternative to ionic liquids in metal-catalysed reactions?

This microreview tries to answer the question: Are Deep Eutectic Solvents a real alternative to Ionic Liquids in metal-catalysed reactions? We have gathered the outstanding results of the use of DES in metal-catalysed processes and we have compared them to the ones with ILs. The huge possibilities of combinations of the components of the DES provide a vast number of different solvents with tunable properties. These DES have been successfully applied to different reactions such as, couplings or hydrogenations, among others. But additional work is still be necessary to explore the use of DES in other reactions and to go in depth in the study of the influence of the nature of the DES. Finally, we also present the rise in a new family of solvents, bio-based ionic liquids, complementary to the use of DES in order to fully substitute traditional ILs.     

Synthesis,structure and catalysis/applications of <Emphasis Type="Italic">N</Emphasis>-heterocyclic carbene based on macrocycles

It is well-recognized that N-heterocyclic carbenes (NHCs) ligands have provided a new dimension to the design of catalysts. Macrocyclic molecules are a class of material chemistry and have served as the synthetic hosts, and molecular recognition. In recent years, researchers have moved toward fabricating interlocking molecules with specific structures and properties. Therefore, researchers have developed more macrocycles complex based NHCs with multi-complexation modes that provide more diverse host–guest systems. In this account, this review highlights recent advances on synthesis, structure and applications of NHC based on macrocycles. According to the structure of different macrocycles, the complexes can be divided into four parts: (i) complexes of NHC based on crown ether; (ii) complexes of NHC based on porphyrin; (iii) complexes of NHC based on calixarene; (iv) complexes of NHC based on the other macrocycles. The complexes showed good coordination ability with different metal ions and showed excellent catalyst activity and optical ability.     

Novel Brønsted acidic deep eutectic solvent as reaction media for esterification of carboxylic acid with alcohols

New halogen-free Brønsted acidic deep eutectic solvents (DES) have been prepared by mixing new quaternary ammonium methanesulfonate salts with p-toluenesulfonic acid (PTSA). They have been used as dual solvent-catalyst for esterification of several carboxylic acids with different alcohols with a reagent molar ratio of 1:1. The method is mild, safe, and simple. Ease of recovery and reusability of DES with high activity makes this method efficient and eco-friendly. The tunability of DES properties, attained by changes in the cation, was performed in order to achieve various esters in good yields.     

Deep Eutectic Solvents Based on Natural Ascorbic Acid Analogues and Choline Chloride

Deep eutectic solvents (DES) are one of the most promising green technologies to emerge in recent years given their combination of environmentally friendly credentials and useful functionalities. Considering the continued search for new DES – especially those that exemplify the aforementioned characteristics, we report the preparation of DES based on natural analogues of l -ascorbic acid for the first time. The onset of eutectic melting occurred at temperatures far below the melting point of the individual components and resulted in the generation of glass forming fluids with glass transition temperatures, viscosities and flow behavior that are comparable to similar systems. This work expands the current array of DES that can be produced using naturally occurring components, which given their potential to be bio-derived, interesting physicochemical properties (e. g. propensity to supercool and vitrify) and apparent antibacterial nature, may provide utility within a range of applications.     

Structure–Activity Relationship of the Thiacalix[4]arenes Family with Sulfobetaine Fragments: Self-Assembly and Cytotoxic Effect against Cancer Cell Lines

Regulating the structure of macrocyclic host molecules and supramolecular assemblies is crucial because the structure–activity relationship often plays a role in governing the properties of these systems. Herein, we propose and develop an approach to the synthesis of the family of sulfobetaine functionalized thiacalix[4]arenes with regulation of the self-assembly and cytotoxic effect against cancer cell lines. The dynamic light scattering method showed that the synthesized macrocycles in cone, partial cone and 1,3-alternate conformations form submicron-sized particles with Ag⁺ in water, but the particle size and polydispersity of the systems studied depend on the macrocycle conformation. Based on the results obtained by ¹H and ¹H-¹H NOESY NMR spectroscopy and transmission electron microscopy for the macrocycles and their aggregates with Ag⁺, a coordination scheme for the Ag⁺ and different conformations of p-tert-butylthiacalix[4]arene functionalized with sulfobetaine fragments was proposed. The type of coordination determines the different shapes of the associates. Cytotoxic properties are shown to be controlled by the shape of associates, with the highest activity demonstrated by thiacalix[4]arenes in partial cone conformation. This complex partial cone/Ag⁺ is two times higher than the reference drug imatinib mesylate. High selectivity against cervical carcinoma cell line indicates the prospect of their using as components of new anticancer system.     

Physicochemical Characterization and Simulation of the Solid–Liquid Equilibrium Phase Diagram of Terpene-Based Eutectic Solvent Systems

The characterization of terpene-based eutectic solvent systems is performed to describe their solid–liquid phase transitions. Physical properties are measured experimentally and compared to computed correlations for deep eutectic solvents (DES) and the percentage relative error e_r for the density, surface tension, and refractive index is obtained. The thermodynamic parameters, including the degradation, glass transition and crystallization temperatures, are measured using DSC and TGA. Based on these data, the solid–liquid equilibrium phase diagrams are calculated for the ideal case and predictions are made using the semi-predictive UNIFAC and the predictive COSMO RS models, the latter with two different parametrization levels. For each system, the ideal, experimental, and predicted eutectic points are obtained. The deviation from ideality is observed experimentally and using the thermodynamic models for Thymol:Borneol and Thymol:Camphor. In contrast, a negative deviation is observed only experimentally for Menthol:Borneol and Menthol:Camphor. Moreover, the chemical interactions are analyzed using FTIR and ¹H-NMR to study the intermolecular hydrogen bonding in the systems.     

Synthesis and studies of crowned dipyrromethenes based macrocycles

Three different meso-substituted new type of crowned dipyrromethene macrocycles were synthesized in decent yields over a sequence of four simple reaction steps using readily available precursors. The macrocycles were thoroughly characterized by HRMS as well as 1D and 2D NMR spectroscopic techniques and the structures of one of the macrocyles in its neutral form and protonated form were determined by X-ray crystallography. The crystal analysis indicated that the macrocycle was distorted in both neutral and protonated forms owing to the presence of flexible aliphatic ether chain. Absorption and electrochemical studies indicated that the properties of macrocycles were dependent on type of aryl group present at the meso-position of dipyrrin moiety of crowned dipyrromethene macrocycle. The preliminary studies indicated that the crowned dipyrromethene macrocycles can act as specific colorimetric optical sensor for Cu²⁺ ion.     

Synthesis,alkali metal picrate extraction,and alkali metal cation binding selectivities of some new cage‐annulated polyoxamacrocyclic crown ethers

Five new cage‐annulated crown ethers, i.e., 4a, 4b, 6b, 11a, and 11b, have been synthesized and their respective alkali metal picrate extraction profiles along with that of a previously synthesized host molecule, 6a, have been obtained. These results are compared with the corresponding results obtained for electrospray ionization mass spectrometric (ESI‐MS) measurements of relative binding selectivities displayed by the same hosts toward a series of alkali metal chlorides. Among the crown‐5 hosts studied, 6a displays enhanced avidity toward complexation with K⁺ picrate in liquid‐liquid extraction experiments. Among the three crown‐6 hosts, 4b proved to be the best alkali metal picrate extractant and displayed significant levels of avidity toward complexation with the larger alkali metal cations (i.e., K⁺, Rb⁺, and Cs⁺). The trends in the picrate extraction and the ESI‐MS results obtained herein show several notable similarities and some differences. The similarities generally stem from size‐selective binding properties that are intrinsic to the different cavity sizes of the cage‐annulated macrocycles, whereas the differences reflect the important influence of solvation effects on the binding properties of the macrocycles.     

Folding-controlled assembly of ortho-phenylene-based macrocycles

The self-assembly of foldamers into macrocycles is a simple approach to non-biological higher-order structure. Previous work on the co-assembly of ortho-phenylene foldamers with rod-shaped linkers has shown that folding and self-assembly affect each other; that is, the combination leads to new emergent behavior, such as access to otherwise unfavorable folding states. To this point this relationship has been passive. Here, we demonstrate control of self-assembly by manipulating the foldamers'' conformational energy surfaces. A series of o-phenylene decamers and octamers have been assembled into macrocycles using imine condensation. Product distributions were analyzed by gel-permeation chromatography and molecular geometries extracted from a combination of NMR spectroscopy and computational chemistry. The assembly of o-phenylene decamers functionalized with alkoxy groups or hydrogens gives both [2 + 2] and [3 + 3] macrocycles. The mixture results from a subtle balance of entropic and enthalpic effects in these systems: the smaller [2 + 2] macrocycles are entropically favored but require the oligomer to misfold, whereas a perfectly folded decamer fits well within the larger [3 + 3] macrocycle that is entropically disfavored. Changing the substituents to fluoro groups, however, shifts assembly quantitatively to the [3 + 3] macrocycle products, even though the structural changes are well-removed from the functional groups directly participating in bond formation. The electron-withdrawing groups favor folding in these systems by strengthening arene–arene stacking interactions, increasing the enthalpic penalty to misfolding. The architectural changes are substantial even though the chemical perturbation is small: analogous o-phenylene octamers do not fit within macrocycles when perfectly folded, and quantitatively misfold to give small macrocycles regardless of substitution. Taken together, these results represent both a high level of structural control in structurally complex foldamer systems and the demonstration of large-amplitude structural changes as a consequence of a small structural effects.

The folding propensity of ortho-phenylene foldamers dictates the outcome of their self-assembly into macrocycles.     

Supramolecular organization and thermal properties of polypseudorotaxanes based on α-cyclodextrine and an ionogenic polymer

The structure, morphology, and thermal properties of polymeric complexes, initial components, and new ionic polypseudorotaxane based on poly(2-acrylamido-2-methylpropanesulfonic acid),-N, N-dimethyl-N??-(4-nitrophenyl)-decane-1,10-diamine-??-cyclodextrine containing photophysically active groups in side chains are studied by X-ray diffraction analysis, scanning electron microscopy, DSC, and TGA methods. It is shown that supramolecular inclusion complexes that give rise to a crystalline precipitate with the columnar structure and hexagonal packing of macrocycles in the base plane are formed. It is found that the structure of complexes is affected by temperature. The models of supramolecular ordering in the complexes that relate the structure and thermophysical properties of the studied systems are suggested.     

Synthesis of a new 24-membered tetramide macrocycle and X-ray crystal structure determination

The synthesis and characterization of a new 24-membered tetramide macrocycle (6) related to Leigh's macrocycles and catenanes is reported. The replacement of p-xylylenediamine (Leigh) by m-xylylenediamine (this work) strongly modifies the geometry and properties of the new macrocycle. NMR spectroscopy (in DMSO?d₆ solution) and X-ray crystallography have been used to characterize compound 6. The structural features in the crystal (conformational aspects and H-bonding) have been discussed comparatively to two similar macrocycles NEWHIJ and UJUNOC.     

Transformation of Type III to Type II Porous Liquids by Tuning Surface Rigidity of Rhodium(II)-Based Metal-Organic Polyhedra for CO2 Cycloaddition

Porous liquids (PLs), a summation of porous hosts and bulky solvents bestowing permanent cavities, are the prominent emerging materials. Despite great efforts, exploration of porous hosts and bulky solvents is still needed to develop new PL systems. Metal-organic polyhedra (MOPs) with discrete molecular architectures can be considered as porous hosts; however, many of them are insoluble entities. Here we report the transformation of type III PL to type II PLs by tuning the surface rigidity of insoluble MOP, Rh₂₄L₂₄, in a bulky ionic liquid (IL). Functionalization of N-donor molecules on Rh−Rh axial sites ensue their solubilization in bulky IL which confer type II PLs. Experimental and theoretical studies reveal the bulkiness of IL as per the cage apertures, and the cause of their dissolution as well. The obtained PLs, capturing more CO₂ than neat solvent, have depicted higher catalytic activity for CO₂ cycloaddition compared to individual MOPs and IL.     

Influence of Hydration on the Structure and Interactions of Ethaline Deep-Eutectic Solvent: A Spectroscopic and Computational Study

Deep-eutectic solvents (DESs) are regarded as alternative green solvents to ionic liquids. In this work we report the structural properties and hydrogen bonding (H-bonding) interactions of an aqueous DES system. The used DES, ethaline (ETH), is composed of choline chloride and ethylene glycol (EG) in 1 : 2 molar ratio. The investigations were carried out by FTIR spectroscopy combined with quantum chemical calculations. Excess spectroscopy and two-dimensional correlation spectroscopy (2D-COS) were used to explore the data in detail. The results showed that, upon mixing, ETH transforms to EG dimers and trimers and D₂O clusters transform to various ETH-D₂O complexes. Theoretical calculations show that water molecules insert between the anion and cation of ETH, break the strong doubly ionic Cl^{− …} H−O_Ch+ H-bond, share charges of the ions and form H-bond with them, thus modulate the interaction properties of ETH. This study deepens our molecular-level understanding of the system and would shed light on its applications.     

Synthesis and properties of π-conjugated donor–acceptor macrocycles derived from phenanthrylene building blocks

Phenanthrylene-ethynylidene macrocycles combining electron donor and electron acceptor subunits in their shape-persistent fully conjugated core were synthesized. The donor subunits consisted of two 9,10-dialkoxyphenanthrenes linked either with 1,2-ethynylidene or 2,5-thienylene bridge. The acceptors were 9,10-phenanthroquinone and dibenzoquinoxaline and dibenzophenazines derived from it. Solvatochromic photoluminescence from intramolecular-charge-transfer (ICT) excited state was observed mainly in non-polar solvents. In more polar solvents, the excited states favor non-radiative relaxation. DFT calculated HOMO/LUMO energies of the macrocycles correlate well with spectroscopic and electrochemical data. In the series of substituted dibenzophenazine acceptors a good correlation with Hammett substituent constants σ_p^– was found.     

An Organic Molecular Nanobarrel that Hosts and Solubilizes C60

Organic cages have gained increasing attention in recent years as molecular hosts and porous materials. Among these, barrel-shaped cages or molecular nanobarrels are promising systems to encapsulate large hosts as they possess windows of the same size as their internal cavity. However, these systems have received little attention and remain practically unexplored despite their potential. Herein, we report the design and synthesis of a new trigonal prismatic organic nanobarrel with two large triangular windows with a diameter of 12.7 Å optimal for the encapsulation of C₆₀. Remarkably, this organic nanobarrel shows a high affinity for C₆₀ in solvents in which C₆₀ is virtually insoluble, providing stable solutions of C₆₀.     

Coronarenes: recent advances and perspectives on macrocyclic and supramolecular chemistry

Synthetic macrocyclic host molecules always play an essential role in the establishment and development of supramolecular chemistry. Along with the continuous interests in the study of classical macrocycles, recent decades have witnessed the emergence and rapid development of the chemistry and supramolecular chemistry of novel and functional macrocycles. Owing to their easy availability, a self-tunable V-shaped cavity resulted from 1,3-alternate conformation, and diversified electronic features steered by the interplay between heteroatom linkages and aromatic rings, heteracalixaromatics act as a type of versatile and powerful macrocyclic hosts in molecular recognition and fabrication of supramolecular systems. Very recently, by means of engineering the bond connectivity or the recombination of chemical bonds within heteracalixaromatics, we have devised coronarenes, a new generation of macrocycles. In this concise review, macrocyclic and supramolecular chemistry of coronarenes are summarized in the order of their syntheses, structural features, molecular recognition and self-assembly properties. In the last part of this article, personal perspectives on the study of macrocyclic and supramolecular chemistry will also be discussed.     

Extraction and Stabilization of Betalains from Beetroot (Beta vulgaris) Wastes Using Deep Eutectic Solvents

Deep eutectic solvents (DES) using magnesium chloride hexahydrate [MgCl₂·6H₂O] and urea [U] proportions (1:1) and (2:1), were prepared for their use as extracting and stabilizer agents for red and violet betalains from beetroot (Beta vulgaris) waste. The synthetized DES [MgCl₂·6H₂O] [U] showed similar properties to eutectic mixtures, such as, liquid phase, low melting points and conductivity, thermal stability, and variable viscosity. In turn, betalain DES extracts (2:1) exhibited compatibility in the extraction and recovery of betalains from beetroot wastes, showing a betalain content comparable to that of betalain extracts. Betalain stability was determined by degradation tests; the exposure conditions were visible light (12 h), molecular oxygen from atmospheric air and environmental temperature (20–27 °C) for 40 days. The kinetic curves of the betalain degradation of water samples depicted a first-order model, indicating the alteration of a violet colouration of betalains from beetroot waste for 5–7 days. However, betalains from DES extracts were kept under visible light for 150 days, and for 340 days in storage (amber vessels), achieving a stability of 75% in comparison with initial beet extracts.     

Coupling the capabilities of different complexing agents into deep eutectic solvents to enhance the separation of aromatics from aliphatics

(Liquid + liquid) extraction of ethylbenzene from n-octane by using tetrabutylammonium bromide-based deep eutectic solvents (DESs) containing pyridine, ethylene glycol, or a mixture of both complexing agents was investigated at 25 °C and atmospheric pressure. The performance of each DES was determined from the distribution ratio and selectivity values calculated using experimental (liquid + liquid) equilibrium data of the ternary systems ethylbenzene + n-octane + DESs. The DES with only ethylene glycol had a high selectivity but a low distribution ratio, whereas the DES with only pyridine had a high distribution ratio but a low selectivity. For the other DESs, adding pyridine increased the distribution ratio, and increasing the molar ratio of ethylene glycol increased the selectivity. Generally, whenever the selectivity increased, the distribution ratio decreased, and vice versa. The nonrandom two-liquid model was used to correlate the experimental data, and the average root mean square deviation (RMSD) between correlated and experimental tie lines was 1.4%. Moreover, the Conductor-like Screening Model for Real Solvents was successfully used to predict the ternary tie lines for the studied systems with an average RMSD of 3.7%.     

Novel triptycene-derived hosts: synthesis and their applications in supramolecular chemistry

The development of new classes of macrocyclic hosts has always been one of the most important topics in supramolecular chemistry. During the past several years, based on the triptycene with unique three-dimensional rigid structure, several different kinds of novel triptycene-derived hosts including triptycene-derived cylindrical macrotricyclic polyether, triptycene-derived tris(crown ether)s, triptycene-derived molecular tweezers, triptycene-derived calixarenes, triptycene-derived heterocalixarenes, triptycene-derived tetralactam macrocycles and molecular cage have been designed and synthesized. Then, by exploring the applications of some of the triptycene derived hosts in molecular recognition and molecular assemblies, a series of new supramolecular systems with specific structures and properties have been developed. This feature article highlights our recent advances in the synthesis of triptycene-derived hosts and their applications in supramolecular chemistry.